Coated metallic article for use at elevated temperatures



Patented Nov. 3,1942

comm) METALLIC narrow FOR USE A1 ELEVATED rmurnaaruans Morand D. Lucas, Cleveland, Ohio No Drawing.' Application {Fume 8, 1946, Serial No. 33938? 6 Claims. (Cl. 117-53) This invention relates to the protection ofmetallic. articles against scaling and oxidation at elevated temperatures. It is among the objects of the invention-to provide metallic articles, especially ferrous metal articles, with a surface coating which is refractory, is resistant to scaling and oxidation at high temperatures in voxidizingatmospheres, is cheap 7' and easily produced from readily available materials, and greatly increases the life of metallic articles exposed to oxidation at elevated temperatures.

A particular object is to provide such a coating which is coherent and is strongly bonded, or

adherent, to the underlying metal, an'd which is resistant to destruction by the stresses'to which the article is subjected in use. v I

Other objects of the invention are to provide" a method of producing coatingsof the type referred to, and to provide materials for making be understood by thoseskilled in the art. Anyor the standard cast iron enamel ground coats may be used in preparing the mixtures used in the practice of the invention. For many purposes I now prefer to use the ground coats used in wetprocess cast iron enameling, which: are

sometimes called bisque ground coats. These adhere satisfactorilmas I-have found,'to both iron and steel inboth sheet and cast form, and

10 when used in accordance with the invention provide coatings which satisfactorily protect the metal without attacking. it further. even at 2100 F. Coatings made using bisque, or wet process cast iron, gl'O'lllldaOOfltS ailord equally l satisfactory protection at both moderately ele- 2 ately elevated'temQiatures, e. g. at 1000 to such coatings and for practicing the method provided by the invention.

Yet another object is' to provide a method of operating ferrous metal articles at elevated temperatures whereby oxidation and scaling-are repressed and the life of such articles is thereby prolonged.

Yet other objects will appear from the fol-' lowing description. v

The invention is predicated upon my discovery that its stated objects are attained by fusing on the surface of the article which is to be protected a coating of'amixture of cast iron enamel ground coat, at least one non-acidic scale-resisting refractory material which when bonded by the fused ground coat renders the coating refractory and resistant to scaling. and silica. Preferably; nickel oxide .is'used as the vated and high temperatures and are therefore preferred for most purposesin the practice of this invention, although their composition may needto be adjusted somewhat for use at moder- 1500 or1600 F., as will appear.

' -Most of the cast iron ground coats are free from or very low incobalt oxide. This is desirable where the article is to be used at more than moderately elevatedv temperatures, say above about l500 or 1600 F. They are desirable also for service in which-the coated article is stressed or shocked to any great extent. The ground coats commonly used with sheet steel or sheet 9 iron are less desirable for coating articles which are used atmore thanmoderately high temperatures, say above 1500" F., because they seem to exhibit a progressive action upon the under- I lying metal when exposed for long periods at such temperatures.

scale-resisting protective refractory, either alone or with another non-acidic refractory, or for less severe service an oxygen compound of chromium, such as chrome ore, may be used. 7 Formost purposes it is advantageous to use both nickel oxide and chrome ore, although other non-acidic refractory materials such as" aluminum oxide,

with nickel oxide in place of the chrome ore.

The cast iron enamel ground coats used inthe practice of the invention themselves afford little resistance to scaling, for example of iron and steel articles, at elevated temperatures, as actual 'tests' have demonstrated. I have found, however, that they serve admirably, particularly when their refractoriness is increased by admixture with silica, as a matrix, or vitrified medium for suspending substantial amounts of scaleresisting refractory materials which act to preburned magnesite, bauxlte, diasporekaolin, fire clay and similar refractory materials may be used The term cast iron ground coat" is used herein for brevity to refer to the well-recognized class of usedand readily available wherever enamellng operations are performed, so that the term will 5 vent or" reduce oxidation and scaling of metals, e. g. iron and steel, covered by such coatings,

whereby the'dife of such articles under exposure to elevated temperatures is increased greatly.

As indicated above, the best results are oh.- I ound coat enamel compositions used for cast iron and steel (see, for example, Enamels" by A. I. Andrews, pages 188-94). These are widely tained with coatingmaterials made from enamel ground coat, preferably bisque ground coat, and substantial amounts of nickel oxide and chrome ore or other similaroxygen compound of chromium. Chrome ore is Preferred, however, be-

cause it produces particularly refractory coat ings, Most suitably the chrome ore used is one which contains from 50 to 60 per cent of chromic oxide. While the exact reason for this is not certain, I now believe that this is due in part to reaction of the nickel oxide, perhaps with silica or with chrome, and to the substantial amounts of magnesia and alumina which occur in most chrome ores.

The proportions in which the enamel ground 'coat, scale-resisting refractory material such as dizing atmospheres than where the coated article is used under less severe scaling conditions. Also, the specific examples given hereinafter will indicate clearly to those skilled in the art how this is to be done. Again, resistance to scaling is increased generally, as my tests have shown, by progressive increase in the amounts of nickel oxide, or of nickel oxide and/or other scale-resisting refractory material, and of silica relative to the amount of ground coat, although the coating composition should not contain too large a proportion of nickel oxide or other similar refractory material because the resultant fused coating may thereby be mechanically weak. I

have found, for'example, that for many purposes the use of from 2 to 4 pounds of nickel oxide per gallon of enamel ground coat slip will produce a satisfactory fused coating that is mechanically strong and highly resistant to scaling of the underlying metal.

When compositions comprising mixtures of cast iron enamel ground coat, silica, and, for example, nickel oxide and chrome ore are fired on a metallic base the ground coat fuses and the nickel oxide and chrome 'ore become dispersed throughout the glassy medium thus formed. Upon cooling the fused ground coat-solidifies and forms a matrix for the other ingredients which act to protect the underlying metal in much the same manner asoxide formed on' the stable surface chromium and chromium-nickel irons and steels acts to prevent'oxidation and scaling at high temperatures. In addition to thus holding the protective, or scale-resisting, refractory material in place to exert its anti-scaling function, the ground coat serves also as a coating impervious to oxygen and carbon, but as noted above, this alone is insufficient protection for the metal. The ground ,coat cooperates, however, with the protective refractory to greatly reduce oxidizing attack of the metal. Tests have shown, for example, that the life of parts exposed to high temperatures may be increased as much as 3.00 per cent in this way. Hence plain carbon partsmay be caused to have the life of more expensive alloy steels.

From what has been said it willbe understood that an important factor is the provision of vitrified coatings that are durable and will resist cracking and spalling during use of the protected article. Nickel oxide has a high coefficient of expans on, and the use of large amounts may cause the coefilcient of expansion of the vitrified c ating to be so different from that of the base as to create a tendency for the-coating to crack or spall, for example under repeated heating or cooling. For that reason silica, preferably as quartz, is added, its low coefficient tending'to offset the increase that would otherwise be caused by the scale-resisting, or protective, refractory material. Preferably the silica is used in an amount to compensate for the high coefficient of nickel oxide so that the coefficient of the vitrified coating will approach that of iron. Generally speaking, one pound of silica is equivalent to two pounds of nickel oxide for this purpose. Silica is useful also because it increases the refractoriness of the coating.

As an example, a mixture, useful for coatin ferrous metal articles such as annealing box covers and other parts exposed to high temperatures comprises a mixture of 8 pounds of bisque ground coat, 2 pounds of black nickel oxide, and 1 pound however, the composition is worked up with water to provide a slurry which may be applied with a brush, or to provide a slip of consistency appropriate to application by dipping, spraying or slushing. An important factor is the forming of a relatively thick layer of the mixture, to form .a thick vitrified coating. After the'composition has been applied to the article it is dried, if need be, following which it is fired at a temperature such as to fuse the composition to the underlying metal. For most purposes firing at temperatures of about 1600 to 1900 F., depending upon the composition of the coating material,'wi1l sufflce. The time of firing will vary according to the mass of the metal and to allied factors, but

generally longer firing is necessary than in ordi-' nary enameling operations, say 30 minutes to 1 hour at 1650 F. In any case, the firing is conducted at a temperature and for a time to fuse the composition satisfactorily on its metal base.

. When using bisque ground coat the article should be placed in a furnace which is at firing heat, or at least heated ,well, toward it, rather than to heat it up in a cold oven, to obtain the best adherence. 1

Although the invention is applicable to the protection against scaling and oxidation at elevated temperatures of articles of various metals and for various uses, it is particularly applicable to the treatment of ferrous metal articles such, for

example, as annealing boxes and covers, oven parts, retorts, crucibles, normalizing furnace parts, carburizing boxes, zinc and salt baths, and the like. For maximum adherence and maximum protection against scaling it is preferred to thoroughly remove scale and the like from the surface of the article prior to applying the coating composition. as by pickling the article or sand blasting the surface which is to be coated.

The fused coatings produced in the practice of the invention are coherent, adherent, and highly resistant to scaling and oxidation of the underlying metal, and they are of sufficient mechanical strength to minimize disruption of the coating under shock andother stresses to which articles,

' such as annealing boxes and covers and the like, maybe normally subjected. Micro-examination of coatings produced in the practice of the invention indicates the strong adherence to be due to intergranular penetration of the coating, and there is reason to believe that in th firing operation some portion of the coating composition is actually reduced to elemental metal which may alloy with the underlying metal and serve to improve the adherence between the latter and the fused coating. Be this as it may, the preferred-compositions are resistant to spallin'g upon bending of the coated article although where the coating does break or spall the surface has an enameled appearance and offers continued resistance to scaling.

The benefits to be derived from the invention have been demonstrated amply by both laboratory and actual commercial tests. In one series of laboratory tests there was used a coating com- .position comprising 12.5 parts by weight of bisque ground coat, 2 parts by weight of powdered nickel oxide, and 1 part by weight of powdered chrome ore. This mixture was thoroughly stirred with water to provide a mix of the consistency of paint so that it could be applied with a brush.

unprotected halt failedeventually, but at that time the treated half retained almost its original thickness.

Pyrometer tubes immersed in a saltbath used. in one plant lasted about 5 days before failing.

When the tubes were treated according to the invention theirlifewas 34 days.

The c positions may be mixed dry and then mixed with water when they are to be used. or, if preferred. there may be used ground coat slips, the ingredients which characterize the invention being added when coating work is to be done. Or, chrome ore may be milled in the enamel slip, if desiredybut the nickel oxide should not. be added until shortly before use because it tends to settle out and harden.

As indicated above, the compositions described specifically should be fired on the articles at temperatures of at least about 1600" F. Once fired on they will protect the articles at lower or I higher temperatures. Forsomepurposcs, howcreased oxidation ofthe underlying steel about thirteen to about fifteen times as compared with uncoated specimens of the pickled steel, and

about seven times in the case of the black sheet. Th tests demonstrate, therefore, the great resistance toscaling provided through the prac ."ice of the invention as well as the benefit to be results consisted of a mixture in water of about I 13 pounds of a standard enamel ground coat,

from 1 to 3 pounds of nickel oxide, and from A to 1 pound of chrome ore.

Still another composition which may be used for some purposes-consists of about 11 pounds of standard enamel, ground coat milled with 2 pounds of water to which there are added from 3 to 6 pounds of powdered chrome ore. I

Yet another composition may be made by grinding from /2 to 1 pound of ground coat with water, about 10 pounds of powdered chrome ore or chrome ore and nickel oxide being added. The amount of water is desirably such as to provide a'paste which may be applied by brushing.

In yet another test a coating composition in accordance with the invention was applied to an annealing cover used in commercial practice at 1600" to 1675" F. Untreated covers scale so rapidly in this particular plant that their lifemay be only four heats, and at best it does not exceed thirteen heats. The cover provided with a coating in accordance with this invention after thirteen. heats and more than 3000 hours exposure was apparently in as good condition as when it was first placed in service, and there is no indication that this cover will fail at any predictable time in the future.

Steel inner'annealing covers used in a strip mill were treated according to the invention.

ever, it may not be feasible to fuse the coatings at such temperatures, or for reasons of economy it-may he wished to fire at lower temperatures.

For example, with parts used at, say, 1100" to' 1400" or 1500? F. it may be desired to fire the coating on in the furnace in which the part is used. Satisfactory results may be had in such cases by modifying'the cast iron enamel coating composition by adding cobalt oxide or molybdic .ox-ide, which cause the material to become satis- -factorily fused and bonded to. the article at such moderately elevated temperatures. Such modified compositions, which are thus like sheet iron and sheet steel ground coats, should not be used with articles that are normally exposed. at 16009 F. or higherin regular use. In the use of these modifiedcoatings removal of mill scale or the like from the article is not necessary because they appear to fuse through it to the metallic base September 21, 1939.

' According to the provisions of the patent statof practicing my invention, and havedescribed utes I have explained the principle and method what I now consider to represent its bestembodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

. 1. That method of treating a metallic article to render it resistant to oxidation at elevated temperatures which comprises app i to said article a coating of an unfritted mixture of chrome ore and cast iron enamel ground coat frit substantially free from cobalt compounds, and heating the coated article to vitrify said frit and form a vitrified coating bonded to the under-- lying -metal, said chrome ore being dispersed through said vitrified frit coating which acts as a matrix therefor, and the coating thus formed protecting the underlying metal against oxidation and scaling at high temperatures.

2. A method according toclaim 1, said mixture containing also nickel oxide and silica in an amount to compensate for'the high expansion coefficient of said oxide, and said oxide being likewise suspended in the vitrified frit coating.

3. A composition for coating metal articles to render them resistant to oxidation and scaling at elevated temperatures comprising an unfritted mixture of a major proportion by weight or cast iron enamel ground coat frit substantially free from cobalt compounds, and a substantial proportion by weight of chrome ore.

4. A composition according to claim 3, said composition containing also substantial amounts of nickel oxide and silica.

5. As a new article of manufacture, a metal article adapted for use at elevated temperatures under oxidizing conditions, said article having its exposed surface provided with an adherent and coherent fused protective coating of cast iron enamel ground coat substantially free from cobalt compounds and having dispersed therethrough and suspended therein, as a matrix, a substantial proportion of chrome ore.

6. An article according to claim 5, said coating containing silica in excess of the amount I present in the frit forming said coating, and the coating having also suspended therein nickel oxide.

MQRAND D. LUCAS. 

